1. Field of the Invention
The invention relates generally to an optical coherent radiation source, and more specifically to semiconductor lasers, fiber optic and integrated optic lasers with enhanced output power used to generate a powerful optical signal in optical fiber communication systems, sensors and other optical devices for industrial and medical applications.
2. Information Disclosure Statement
Semiconductor lasers, integrated optical and optical fiber lasers are becoming key components of optical fiber communication systems, sensors, fiber-gyros and other optical devices for various industrial and medical applications. All these lasers essentially consist of an active waveguiding core region and outer cladding with lower refractive index In the case of semiconductor lasers an electric current is used to pump the active region of p-n junction. In the case of optical fiber lasers and integrated optical lasers a rare-earth-doped optical glasses pumped with an external light source such as a single-mode semiconductor laser or diode-laser array are usually employed. Mirrors or flat end faces of the waveguide can be used to provide the system with the necessary feedback.
Most fiber lasers are based on conventional high-purity silica-based optical fibers manufactured by chemical-vapor-deposition (CVD) and are doped, for example, with such rare-earth elements as Nd.sup.3+, Er.sup.3+, Tm.sup.3+, Ho.sup.3+, Yb.sup.3+, Sm.sup.3+, and Pr.sup.3+ via gas-phase or solution-doping processes. The same materials can be used in integrated optical lasers based on planar waveguiding structures.
Single-mode active fibers or waveguides are usually used in these lasers because they can easily be integrated into single-mode fiber-communication and sensor networks. Primary disadvantage of such single-mode fiber or integrated optical lasers is the difficulty of coupling the pump light from a diode laser array into the active waveguide core region whose cross section is not much larger than the wavelength of the pump light. Even using a diffraction-limited semiconductor laser as the pump source fails to alleviate the problem due to the laser's low output power. The pump light from semiconductor sources usually couples into a highly multimode doped cladding region which guides the pump radiation. Thus, amplification of the light takes place only in a single-mode rare-earth-doped core while a considerable portion of the mode field propagates in the passive cladding region.
The problem of coupling pump light into an active waveguide core can be solved, however, by using larger core sizes, i.e. multimode waveguide lasers. Such multimode waveguide lasers guiding large amount of transverse modes are expected to be much more powerful because of the more efficient pumping of their active core and more efficient utilization of the pump power. The mirror-coating-damage problem can be solved as well by using larger core sizes as found in multimode active waveguides.
Directly employing the multimode active waveguide instead of a single mode one does not solve the problem, since an interference of all the waveguide modes results in a complicated speckle pattern at the laser output mirror destroying in such a way the quality of the output laser beam and drastically reducing the efficiency of coupling of this beam into an output single-mode fiber network. Moreover, in the case of semiconductor lasers the generation of many transverse modes results also in an undesired increase in threshold of the pump current. Therefore, it is desirable to have a possibility to provide a single-mode regime of operation of the laser having a large cross section of the active core required for its efficient pumping.